Add new OAuth2 strategy, and deprecate old one

This is a mildly breaking change, and so therefore it revs the major
revision number for this package. Here's what needs to change:

    // Change the Strategy import:
    var FamilySearchStrategy = require('passport-familysearch').Strategy;

    // To LegacyStrategy:
    var FamilySearchStrategy = require('passport-familysearch').LegacyStrategy;

A more involved change is needed to upgrade to the new
OAuth2 strategy (https://familysearch.org/developers/docs/oauth2).
FamilySearch is requesting that all clients move to this new strategy
(https://familysearch.org/developers/docs/authentication).